partialResult of customBuiltin

[Evange95]

Hello everyone, I would like to know if it is possible to partially evaluate a policy rego that uses a custom builtin.
In detail the builtin performs a call to a mongodb collection taking all the parameters needed from the context.
I'm currently having some problems with it and I want to know if it is not possible in general.

Thank you!

[srenatus]

Built-ins in general are partially evaluated as follows:

• if all their arguments are known, and the builtin is not declared impure, it will be evaluated and the result used to further evaluate the quert
• if any of their arguments are unknown, or the builtin is declared impure, a call to the builtin with the arguments (evaluated as far as possible) is saved -- i.e., added to the query or support module

See the difference here, time.now_ns() is unsafe, and startswith() is not:

$ opa eval -fsource -p 'time.now_ns() < 10' # impure function
# Query 1
time.now_ns() < 10

$ opa eval -fsource -p 'startswith("foox", "foo"); input > 10' # inputs known, the func call is evaluated
# Query 1
input > 10

$ opa eval -fsource -p 'startswith(input, "foo")'
# Query 1
startswith(input, "foo")

The list of impure built-in functions is here: https://github.com/open-policy-agent/opa/blob/v0.37.1/ast/builtins.go#L274-L283